Adjustable coil



Feb. 14, 1956 A. c. HUDSON 2,735,076

ADJUSTABLE COIL.

Filed April 25, 1952 Mum/rye 7 2,735,076 Patented Feb. 14, I956 ADJUSTABLE COIL Arthur C. Hudson, Billings Bridge, Ontario, Canada, as-

signor to National Research Council, Ottawa, Ontario, Canada, a body corporate of Canada Application April 25, 1952, Serial No. 284,461 6 Claims. ,(Cl. 336-437) The invention relates to an adjustable helically wound coil such as m'ay'be used in the high frequency circuits of electronic apparatus.

There have been two common methods of adjusting a physically large coil such as may be used, for example, in the tank circuit of aradio transmitter or of an induction heating oscillator. One method is to use an adjustable tap for one connection to the coil, and the other method has been to use an adjustable connection to short-circuit some of the turns at one end of the coil. Each of these methods gives rise to the disadvantage that currents flow in the unused part of the coil, resulting in losses of energy and spurious resonances due to stray capacitances.

According to the present invention, the disadvantages of the prior art are overcome by having the coil divided into two sections of conductor having the same winding pitch and diameter and arranged on acommonaxis. At least a part of the conductor of one of the sections is arranged in telescopic connection with at least a part of the conductor of the other, and the two sections are arranged for relative rotation so as to vary the length of the coil. Preferably the conductor of one section is tubular, having one open end forming part of a turn of the coil, and the conductor of the other section is of such cross-sectional size and shape as to fit helically within the tubular conductor of the first section thereby forming a telescopic connection. Electrical connections. can be made to the ends of the coil in a conventional manner and, if desired, direct electrical contact can be established between the sections of the coil by spring fingers or a set screw. In a preferred form, the movable section is guided by a sleeve arranged axially of the coil and a shaft may be arranged to rotate the movable coil by means of a knob.

Adjustment of a coil according to the present invention does not lead to disadvantages due to currents flowing in the unused portion of the coil as this unused portion throughout its length is in electrical connection with the used portion, either directly or by means of electrical capacitance and, in most forms of the invention, the tendency of high frequency currents to flow near the surface of conductors ensures that there is little if any current in the unused conductor. The electrical capacitance existing between the unused portion of the conductor and the used portion of the conductor which faces it serves to transfer high frequency current from one section of the coil to the other so that even in the case of a poor mechanical connection between the two sections of the coil there exists a good electrical connection for high frequency currents.

The invention will be further described with reference to the accompanying drawing which shows a preferred embodiment of it.

The coil shown in the drawing comprises a fixed section 10 (the lower section of the coil when in the position shown in the drawing) of copper tubing wound helically 2. with spaced turns and an upper section 11 of copper wire wound helically with the same diameter and pitch as the lower section 10. The lower section 10 is fixed at one end to a base plate 12 of an electrically insulating material by a terminal 13 and at the other end' it terminates at a connection 14. The connection 14 is formed by cutting a plurality of slots 15 lengthwise of the copper tubing for a short distance to form fingers 16. The fingers 16 are bent inwardly of the conductor to provide a resilient enclosure of the opening inthe connection 14. A collar 17 is provided near the connection 14 andhas a set screw 18 which is threaded through the collar 17' and extends into the interior of the copper tubing.

The diameter of the wire used as the conductor of the upper section 11 is suchv that the wire slides freely in a telescopic manner through the connection 14 and into the tubing which forms the lower section 10. The upper end of the conductor of the section 11 extends radially to form a radial part 19 and then is bent through a small hairpin bend 20 so that it extends axially of the coil to form an axial part 21. The axial part 21 of the conductor of the section 11 passes into a sleeve 22, which may be copper tubing of the same outer and inner diameters as the copper tubing used as the conductor of the section 10 of the coil. The sleeve 22 is provided with a connection 23 of the same construction as the connection 14 and also is fitted with a collar 17' and a set screw 18. The sleeve 22 is secured to the base plate 12' and is connected to a terminal 24 which extends through the base plate 12.

In order that a knob may be provided for manual adjustment of the inductance of the coil, a plate 25 of electrically insulating material is supported from the base plate 12 at the top of the coil by screws-26 and spacers 27. A shaft 28 of electrically insulating material is'sup ported by a bearing in the upper plate 25 and extends above the upper plate 25 into a knob 29. The shaft 28 extends below the upper plate 25 and has a slot 30 enclosing the hairpin bend 20 and some of the axial part 21 of the conductor of the section 11. Rotation of the shaft 28 by the knob 29 acts upon the radial part 19 of the section 11 so that the section 11 is rotated and passes telescopically into or out of the conductor of the lower section 10.

During adjustment of the inductance of the coil, the set screws 18 are backed off so that they do not bear against the conductor of the upper section 11. Then by rotating the shaft 28 by means of the knob 29 (or a handwheel in the case of a physically large coil) the section 11 is rotated about its axis and the lower portion of its conductor turns within the upper portion of the tubular conductor of the lower section 10 so that the length and the effective number of turns of the coil are reduced, the unused portion being totally enclosed within the conductor of the lower section 10. If it is desired to increase the inductance of the coil, the upper section 11 is rotated so that more of its lower portion is turned out of the upper portion of the conductor of the lower section 10. When the final setting of the inductance of the coil has been obtained, the set screws 18 are threaded into the collars 17 so that they bear against the conductor of the upper section 11, thereby making direct electrical contact between the conductor of the lower section 10 and the conductor of the upper section 11. However, if desired, the upper plate 25, the screws 26, the spacers 27, the shaft 28 and the knob 29 may be omitted and, in this case, with the coil de-energized, the upper section 11 can be turned directly by hand to adjust the inductance of the coil.

If it is desired to have the coil adjustable from a knob or a handwheel located below the base plate 12, the

3 axial part 21 of the upper section 11 may be extended through the sleeve 22 so that the axial part 21 extends through the base plate 12 where it may be fitted with a knob or a handwheel for manual adjustment of the coil. In this latter case electrical connection to the upper section 11 may be made through the sleeve 22.

In another arrangement of an adjustable coil according to the invention, slotted conductor, for example C-shaped conductor, is used for the lower section 10 of the coil and several drive-pins are fixed at intervals along the conductor of the upper section 11 so that the drive-pins can pass along the slot in the conductor of the lower section 10 and extend therefrom. Drum shaped drivemeans is then employed to engage the pins and to rotate the upper section 11 of the coil upon rotation of the drive-means by a handwheel or by a motor.

The conductor of the upper and lower sections 11 and 10of the coil may be of other cross-sectional shapes than that shown in the drawing providing such shapes allow a telescopic connection between the two sections of the coil. For example, the conductor of the lower section 10 may be U-shaped, or C-shaped and the conductor of the upper section 11 may be solid or hollow, square, rectangular or circular. If desired the conductors of both sections could be solid and held in telescopic connection by yokes which would permit the conductor of one section to slide past that of the other.

What I claim as my invention is:

1. An adjustable helically wound coil comprising two sections of conductor having the same winding pitch and diameter and arranged on a common axis, each of said sections of conductor forming spaced turns of said coil, a telescopic connection between at least a part of an individual turn of one of said two sections and at least a part of an individual turn of the other, said tele- 35 scopic connection being in the direction of a turn of said coil and forming an electrical and mechanical connection between the turns of said two sections, means mounting said two sections of conductor for relative rotation of one 4 with respect to the other about said common axis thereby to vary the length of said coil.

2. An adjustable helically wound coil as defined in claim 1 in which the conductor of one section is tubular having one open end forming part of a turn of said coil, the conductor of the other section being of such cross-sectional size and shape as to fit helically within the tubular conductor of said one section thereby forming the telescopic connection between the two sections of conductor of the coil.

3. An adjustable helically wound coil as defined in claim 2 comprising means at the telescopic connection to form a direct electrical connection between the two sections of conductor of the coil.

4. An adjustable helically wound coil as defined in claim 2 in which the end of the conductor of said other section extends radially of said other section and then axially of said coil, said coil comprising sleeve means arranged to guide said end of the conductor during axial movement upon relative rotation of the sections of conductor of the coil.

5. An'adjustable helically wound coil as defined in claim 4 in which the sleeve means is in direct electrical contact with said end of said other section and is connected to terminal means for said coil.

6. An adjustable helically wound coil as defined in claim 4 comprising a rotatable shaft arranged axially of said coil and having a slot running longitudinally from one end, the radially and axially extending parts of said end of said other section being each at least partly within said slot.

References Cited in the file of this patent UNITED STATES PATENTS 1,629,867 Herman May 24, 1927 1,642,350 Adams Sept. 13, 1927 1,904,208 Dow Apr. 18, 1933 2,451,809 Clark Oct. 19, 1948 

